Is it always a bad idea to use grayscale to normal map filters?

Macattackk

So I know that converting a grayscale image to a normal map is technically incorrect because the converted normal map parts do not actually correctly form to the tangents of your low poly model, but is there any way to make this work? Im assuming that when people do this they are just saying "fuck it" and letting it slide even though it is technically an inaccurate way to add parts to a normal map.

The way I'd want to use this and have used this in the past is by adding textural details, like fabric details in photoshop to the diffuse texture then converting that using something like the NVIDIA normal map filter but Im having second thoughts on ever doing that again since it might not be correct. And I guess this also means that using nDo's normal map creation tools is technically wrong as well. What is the general thought and wisdom behind techniques like this? Is it generally better to be a purist about normal maps or is the conventional wisdom that it just doesnt really matter since it doesnt create that huge of a negative impact on the quality of a normal map?

Goat Justice

Its very hard to set absolute rules like "Always do X" or "Never do Y". Better to form an understanding of what each technique is good for. Others may correct me if I'm wrong, but Its been my experience that tangent matching comes into play more when dealing with "Macro" normals that try to create a reshaping effect on the underlying geometry. Normals created from height-maps are often ok for adding small details or high frequency noise to a more or less flat surface. Height map normals can also be layered over macro normals with some amount of success... just kinda depends on the situation and where the texture will ultimately be used.

That's a pretty major simplification, but it's a huge topic.

passerby

there are no hardset rules, just that if you dont bake high to low, the normalmap will be fighting with the shading on the lowpoly mesh.

for flat surfaces, useing ndo2 or what ever works pretty good, i ussualy just tweak it a little and take down the blue channel on added details.

in the end it doesn't have to be all 100% mathematically correct, as along as the end product looks fine.

EarthQuake

If you're adding texture details on top of a baked normal map there really shouldn't be any issues. As long as you're doing it in a way which does not alter the base baked normal map, like leveling the blue channel out at 127 and overlaying it for instance.

Now if you don't have a baked normal map to start with, there is just no way you can "paint" a height map that would ever be as accurate, because as you say it doesn't take the normals and tangents of the lowpoly mesh into consideration.

Macattackk

Yea, like I said its never really made my normal maps look bad in real time, I just thought there might be some unforeseen issues with it. But I guess its more of a macro vs micro details. And its probably better to only layer one or two things on top of a baked normal map and not too much stuff as it might muddy the tangent correct macro details from the bake. And yea most people who play video games won't notice or care about this as long as it is generally pleasing to the eye.